Primary radiation damage of Zr-0.5%Nb binary alloy: atomistic simulation by molecular dynamics method

Abstract

Ab initio calculations predict high positive binding energy (∼1 eV) between niobium atoms and self-interstitial configurations in hcp zirconium. It allows the expectation of increased niobium fraction in self-interstitials formed under neutron irradiation in atomic displacement cascades. In this paper, we report the results of molecular dynamics simulation of atomic displacement cascades in Zr-0.5%Nb binary alloy and pure Zr at the temperature of 300 K. Two sets of n-body interatomic potentials have been used for the Zr-Nb system. We consider a cascade energy range of 2–20 keV. Calculations show close estimations of the average number of produced Frenkel pairs in the alloy and pure Zr. A high fraction of Nb is observed in the self-interstitial configurations. Nb is mainly detected in single self-interstitial configurations, where its fraction reaches tens of percent, i.e. more than its tenfold concentration in the matrix. The basic mechanism of this phenomenon is the trapping of mobile self-interstitial configurations by niobium. The diffusion of pure zirconium and mixed zirconium-niobium self-interstitial configurations in the zirconium matrix at 300 K has been simulated. We observe a strong dependence of the estimated diffusion coefficients and fractions of Nb in self-interstitials produced in displacement cascades on the potential.

Atomistic simulation of diffusion of the self-interstitial atom in HCP Zr

“…The values of formation energies for the different SIA positions in HCP Zr calculated by molecular statics method are shown in table 1. A description of the method for calculating the SIA formation energy can be found elsewhere [19]. The corresponding ab initio results obtained by different researchers are presented in table 1 as well.…”

“…The LWP-2013 potential gives the lowest formation energy for the BC and BS configuration. For the MA-2007 potential, the position with the lowest formation energy of SIA is given in paper [19]. This position lies approximately midway between the C and O configurations.…”

Distribution of niobium atoms in self-interstitial configurations in binary alloys Zr–(0.5–3)%Nb after passing the atomic displacements cascade

“…The configurations with the highest binding energy are BO, BS and BC. It is notable to note that the most energy-efficient SIA configuration in pure Zr is (C+O)/2 [11]. But this configuration is not stable for the case of the interstitial Nb.…”

“…The possible contribution to this phenomenon of processes occurring at the stage of primary radiation damage has not been studied in details. In our previous work [11], we noticed that in Zr-0.5%Nb alloy Nb atoms are actively trapped by interstitial configurations formed in displacement cascade and migrate via an interstitial mechanism. Therefore, it is important to know how fraction of Nb in Zr-Nb alloy affects on evolution of atomic displacement cascades and, in particular, on the formation of interstitial configurations.…”

“…Thus, Zr-(0.5, 1, 1.5, 2, 2.5, 3)%Nb alloys are considered in the present work to identify the influence of Nb on atomic displacement cascade evolution and formation of post-cascade structure. Similar processes in binary Zr-Nb alloys were considered in [11][12][13], but they did not cover a wide range of niobium fraction.…”